Add support for GIT_SUBMODULES argument

[gillamkid]

CPM does passthrough for many of the argument options of cmake's FetchContent() function. Hwever, there was an argument of interest that it was not passing: GIT_SUBMODULES.

GIT_SUBMODULES is needed if you want to use CPMAddPackage() and have it not initialize the submodules of the repo.

I added support so GIT_SUBMODULES "" can be used to prevent submodules from being initialized the same way it is used to do so when calling FetchContent() directly.

[TheLartians]

Thanks for raising the issue! We should definitely support this edge case. Three things

• Can you add a test case so we don't break this accidentally?
• The issue is not specific to GIT_SUBMODULES issue will happen any time someone tries to pass an empty string to a forwarded parameter. Do you think there's a way we can make this a more general fix? E.g. writing a wrapper for cmake_parse_arguments that ensures empty arguments are preserved?
• Please run the code formatter (see CONTRIBUTING.md)

[TheLartians]

@gillamkid note that this issue is also handled by #461, which resolves the issue in a more general way. The PR has gone stale due to lack of time from my side, but given the obvious need for a fix I will prioritise it.

[gillamkid]

I added some unit tests and fixed the styling. @TheLartians can you please approve the Gitlab Actions workflow to run?